In this context, "アレ" doesn't refer to the exclamation "あれ！" (which would be expressing surprise or pointing out something unexpected like "what?"). Instead, it's being used to imply something that's understood in the context of their conversation without specifying what it is.

So when the dad says "アレな服、着ればいいのに" he's suggesting she wear clothes that are more... something. The "アレ" stands in for an adjective that he doesn't specify, but based on context, it implies clothes that are more fashionable, girly, or perhaps more eye-catching than what she's currently wearing. It's a way of saying "You should wear more 'you know what' kind of clothes," something more socially expected or desirable for a girl her age, according to the father's perspective.

This use of "アレ" is quite common in Japanese and can be used in various contexts to imply something known to both speaker and listener without naming it directly, often to avoid being too direct, to be polite, or simply because the speaker assumes the listener can infer the meaning.